Carbon dioxide gas laser machining method of multilayer material

ABSTRACT

In a carbon dioxide laser machining method of a multilayer material of applying carbon dioxide laser light to a machined part of a multilayer material having an insulation layer and a first conductor layer and a second conductor layer deposited with the insulation layer between and removing the first conductor layer and the insulation layer of the machined part to form a blind hole or a groove arriving at the second conductor layer, the laser light is applied to the machined part like pulses at an energy density of 25 J/cm 2  or more for beam ON time in the range of 1 μs to 10 μs.

TECHNICAL FIELD

This invention relates to a carbon dioxide laser machining method of amultilayer material for forming a through hole and a blind hole toelectrically connect a plurality of conductor layers in a multilayerwiring board called a printed wiring board.

BACKGROUND ART

Generally, a printed wiring board is a board having conductor layers 4and 5 made of copper foil on both sides of an insulation layer 1 formedby impregnating glass cloth 3 with resin 2 and hardening, as shown inFIG. 9; a printed wiring board of a board shape made up of multiplelayers as shown in FIG. 10 is also available.

Hitherto, the following two methods for forming a blind hole toelectrically connect the conductor layers 4 and 5 on both sides of theinsulation layer 1 in such a printed wiring board have been available:

The first method is a method using carbon dioxide laser light, whereinthe conductor layer 4 on the laser light incidence side is removed byany other method than the carbon dioxide laser light, such as etching ordrilling, and then only the insulation layer is machined by applyinglaser light using the fact that the carbon dioxide laser light is almostreflected on the conductor layer although it is well absorbed in theinsulation layer 1.

The second method is a method using solid state (YAG, etc.,) laserlight, wherein a blind hole is formed only by applying laser light usingthe fact that the solid state laser light is well absorbed in both theinsulation layer and the conductor layer unlike the carbon dioxide laserlight.

However, to remove the conductor layer by drilling in the first method,it is difficult to make fine adjustment in the depth direction and it isimpossible to stably remove the conductor layer so as not to causedamage to the conductor layer 5 on the bottom. To remove the conductorlayer by etching, there is a problem of increasing the cost because theetching process is complicated.

Further, to remove the conductor layer 4 and the insulation layer 1 byapplying solid state (YAG, etc.,) laser light in the second method,there is a problem of increasing the production cost because the solidstate laser running cost is high.

Considering the problems, at present both the conductor layer 4 and theinsulation layer 1 are machined to form a blind hole only by applyingthe carbon dioxide laser light.

Specifically, to machine the conductor layer based on the carbon dioxidelaser light, reflection of the carbon dioxide laser light on theconductor layer on the laser light incidence side is remarkable largeand thus energy of the carbon dioxide laser light applied to stablymachine the conductor layer is made fairly large as compared with thecase where only the insulation layer is machined.

If carbon dioxide laser light of fairly large energy is applied formachining as described above to reliably remove the conductor layer 4for a printed wiring board as shown in FIG. 11A, while one-pulse laserlight is being applied, the machining is advanced in the order of FIGS.11B, 11C, 11D, and 11E and as shown in 11E, the conductor layer 4 on thelaser light incidence side is projected into a hole, the hole shapebecomes a middle swell shape, or damage to an inner conductor layer 5occurs; this is a problem. The reason why such a problem occurs is thatexcessive heat is input to the insulation layer 1 because the energy ofthe carbon dioxide laser light is made large to machine the conductorlayer 4.

The laser light incidence side where machining is performed, namely, theconductor layer 4 on the surface reflects the carbon dioxide laser lightremarkably largely and the heat input amount and the thermal diffusiondirection after heat input do not become stable and thus the roundnessof a machined hole easily worsens and there is a problem of how toenhance the roundness of a machined hole.

DISCLOSURE OF INVENTION

The invention is intended for solving the problems and it is an objectof the invention to provide a carbon dioxide laser machining method of amultilayer material wherein when a conductor layer on the laser lightincidence side is removed, carbon dioxide laser light is optimized,whereby the conductor layer on the laser light incidence side is stablyremoved and the hole shape does not becomes a middle swell shape.

To accomplish the object, according to a first aspect of the invention,there is provided a carbon dioxide laser machining method of amultilayer material of applying carbon dioxide laser light to a machinedpart of a multilayer material having an insulation layer and a firstconductor layer and a second conductor layer deposited with theinsulation layer between and removing the first conductor layer and theinsulation layer of the machined part to form a blind hole or a groovearriving at the second conductor layer, wherein the laser light isapplied to the machined part like pulses at an energy density of 25J/cm² or more for beam ON time in the range of 1 μs to 10 μs.

When the first conductor layer of the machined part is removed byapplying a plurality of pulses, the application beam diameter of thelater applied pulse is made larger than that of the earlier appliedpulse.

BRIEF DESCRIPTION OF DRAWINGS

FIGS. 1A to 1C are drawing to show the process of machining by a lasermachining method according to a first embodiment of the invention.

FIG. 2 is a drawing to show the removal characteristic of copper foil 12μm thick upon application of laser light.

FIG. 3 is a drawing to show the relationship of machined hole depth withenergy density and beam ON time.

FIG. 4 is a drawing to show the middle swell percentage of hole shaperelative to beam ON time.

FIG. 5 is a drawing to show the removal characteristic of copper foil 12μm thick upon application of laser light by a laser machining method asa comparison example.

FIGS. 6A to 6D are drawing to show the process of machining by a lasermachining method according to a second embodiment of the invention.

FIG. 7 is a drawing to show the roundness of a machined hole relative tothe laser light application area.

FIGS. 8A and 8B are drawing to show the temperature distribution of amachined part.

FIG. 9 is a sectional view of a general printed wiring board.

FIG. 10 is a sectional view of a general multilayer printed wiringboard.

FIGS. 11A to 11E are drawing to show the process of machining by a lasermachining method in a related art.

BEST MODE FOR CARRYING OUT THE INVENTION

First Embodiment.

A carbon dioxide laser machining method of a multilayer materialaccording to a first embodiment of the invention will be discussed withFIGS. 1A to 5.

In the embodiment, a wiring board having conductor layers 4 and 5 on thesurface and back of an insulation layer 1 formed by impregnating glasscloth 3 with resin 2 and hardening is formed with a blind hole toelectrically connect the conductor layer 4 on the laser light incidenceside and the conductor layer 5 on an opposite side.

The glass cloth 3 exists to improve the electric reliability of theboard and the board strength and may be replaced with any other materialand may not necessarily exist.

Here, the printed wiring board machined using the carbon dioxide lasermachining method of a multilayer material according to the embodimenthas the conductor layer 4 being copper foil 12 μm thick, the conductorlayer 5 being copper foil 18 μm thick, and the board 1 being epoxy 80 μmthick as shown in FIG. 1A. The hole diameter of a target blind hole isφ100 μm.

To begin with, as shown in FIG. 1B, as the first pulse, carbon dioxidelaser light 6 with the pulse beam ON (beam application) time being 3 μsand the energy density being 150 J/cm² is applied to the range of areaφ100 μm of the conductor layer 4 of the printed wiring board whereremoval is required, and the conductor layer 4 and some of theinsulation layer 1 are removed.

In the removal, the laser light with the beam ON time ranging from 1 μsto 10 μs and the energy density being 25 J/cm² or more is used and thusas compared with the case where carbon dioxide laser light with the beamON time shorter than 1 μs or carbon dioxide laser light with the beam ONtime longer than 10 μs is applied to the same area at the same energydensity, the energy of the carbon dioxide laser light is efficientlyabsorbed and consumed for removal of the conductor layer 4 and extracarbon dioxide laser light does not machine the insulation layer 1unnecessarily largely, so that it is made possible to prevent theconductor layer 4 from being projected into the hole or the hole shapefrom becoming a middle swell shape upon application of one pulse.

Here, the reason why the energy of the carbon dioxide laser light isefficiently absorbed and consumed for removal of the conductor layer 4and extra carbon dioxide laser light does not machine the insulationlayer 1 unnecessarily largely will be discussed.

FIG. 2 indicates whether or not a conductor layer can be removed whenthe one-pulse beam ON time and the energy density are changed and laserlight is applied to the conductor layer 12 μm thick as application areaφ100 μm; ∘ indicates that the conductor layer can be removed and Xindicates that the conductor layer cannot be removed.

The tendency of ∘ and X in FIG. 2 becomes almost similar when theconductor layer has a thickness in the range of 3 to 12 μm.

The reason is that the absorption ratio of the carbon dioxide laserlight depends largely on the reflection factor of laser light on theconductor layer surface rather than the thickness of the conductorlayer.

It is seen in FIG. 2 that if the energy density is made constant whenthe beam ON time is 1 μs or more, the shorter the beam ON time, the moreenhanced the conductor layer removal capability, making it possible toremove copper foil.

The reason is that a predetermined power density (=energy density÷beamON time) is required for machining the conductor layer 4. (A powerdensity of 10⁷ W/cm² or more is required to remove general copper foil 3to 12 μm thick.)

If laser light with a long beam ON time and a low power density isapplied, the conductor layer 4, namely, copper foil has high thermalconductivity and thus the heat absorbed in the copper foil escapes tothe surroundings and the copper foil cannot efficiently be removed.

It is seen in FIG. 2 that when the beam ON time becomes less than 1 μs,a larger energy density is required to remove the conductor layer ascompared with the case where the beam ON time is 1 μs or more.

Although power density is required to machine the conductor layer 4,since laser light with short beam ON time and too large power density isapplied, it becomes difficult for the heat absorbed in the copper foilto diffuse to the surroundings and only the laser application part(copper foil) is heated excessively.Since the laser light is consumed to excessively raise the temperatureof the laser application part (copper foil), the removal volume perpulse becomes extremely small.

Further, FIG. 3 shows the relationship between the beam ON time and thehole depth of the insulation layer 1 when the one-pulse energy densitywas set to 10, 20, and 30 J/cm² and laser light of application area φ100μm was applied to a board of only epoxy 500 μm thick.

It is seen in FIG. 3 that the longer the beam ON time and the larger theenergy density, the deeper removed the insulation layer 1.

To remove the insulation layer 1 and form a blind hole made blind in theconductor layer 5, since the thickness of the insulation layer 1 islimited, after the removal depth arrives at the thickness of theinsulation layer 1, removal of the insulation layer 1 proceeds in theperpendicular direction to the traveling direction of incident laserlight and thus the blind hole becomes a middle swell shape.If the same energy density is applied, the shorter the beam ON time, thehigher the power density. Thus, before heat diffuses to the laser lightapplication part surroundings, only the laser light application part(resin) is heated excessively and the temperature of the laser lightapplication part becomes high and thus the removal volume per pulsebecomes small and the hole depth becomes shallow.In contrast, the longer the beam ON time, the lower the power densityand thus heat diffuses to the laser light application part surroundings(depth direction), so that the removal volume per pulse becomes largeand the hole depth becomes deep.

In the embodiment, the beam ON time is in the range of 1 μs to 10 μs, sothat the hole depth becomes shallow because of the reason describedabove and after the conductor layer on the surface is removed, it ismade possible to prevent the insulation layer from being removed deepmore than necessary or the blind hole from becoming a middle swellshape.

Further, FIG. 4 shows the relationship between the beam ON time and themiddle swell percentage of hole shape when the one-pulse energy densityis set to 100, 150, 200, and 250 J/cm². The middle swell percentage ofhole shape is calculated as follows:(Middle swell percentage)=100×{(hole diameter of insulation layer)−(holediameter of conductor layer on laser light incidence side)}/(holediameter of conductor layer on laser light incidence side)If the conductor layer projects into the hole or the hole shape does notbecomes a middle swell shape, the middle swell percentage is set to 0(%).It is seen in FIG. 4 that the longer the beam ON time, the higher themiddle swell percentage.This means that if the pulse width is long, the power density becomeslow and therefore the insulation layer is removed deep.Further, it is seen that the larger the energy density, the higher themiddle swell percentage.This means that if the energy density is large, the insulation layer isremoved deep.

It is seen in FIG. 5 provided by integrating the results in FIGS. 2 and4 that when the beam ON time is in the range of 1 to 10 μs, surfacecopper foil can be removed and the middle swell percentage is 10% orless and therefore the beam ON time in the range is appropriate beam ONtime.

However, it seems that the appropriate beam ON time a little changesdepending on the thicknesses of the conductor layer on the surface andthe insulation layer.

JP-A-9-107168 proposes a laser machining method using laser light withthe beam ON time ranging from 10 μs to 200 μs and the energy densitybeing 20 J/cm² or more.

Area of a in FIG. 5 corresponding to the condition described inJP-A-9-107168 is fitted in the point of efficiently removing resin only,but not fitted in machining wherein compatibility between efficientremoval of copper foil and prevention of a middle swell shape isrequired, intended by the specification.

JP-A-10-323777 proposes a laser machining method using laser light withthe beam ON time ranging from 3 μs to 4 μs and the energy density being22 J/cm² or less.

Area shown in b in FIG. 5 corresponds to the condition described inJP-A-10-323777, but in the area, the energy density is low as 22 J/cm²or less and energy density 25 J/cm² to stably remove the conductor layercannot be provided and thus it is not fitted for machining wherein theconductor layer (copper foil, etc.,) needs to be removed efficiently,intended by the specification.

Next, to machine the insulation layer 1 as shown in FIG. 1B left afterthe laser light 6 is applied, after the laser light 6 as the first pulseis applied, laser light 7 as shown in FIG. 1C is applied as the secondpulse and later, thereby machining the remaining insulation layer 1 forcompleting formation of the blind hole.

Here, the laser light 7 may machine only the insulation layer ratherthan the conductor layer and does not require the energy density or thepulse width as indicated in the patent and therefore it is desirablethat laser light with the beam ON time ranging from 10 μs to 200 μs andthe energy density being 20 J/cm² or more as proposed in JP-A-9-107168,etc., for example, fitted for removal machining of the insulation layershould be used.

Further, to prevent the hole shape from becoming a middle swell shape,it is desirable that the energy density should be lessened.

If the energy density is too high, as shown in FIG. 9E, excessive laserlight is reflected by the conductor layer 5 and removes walls of theinsulation layer and thus the hole shape becomes a middle swell shape.

In the embodiment, laser light is set to 15 μs and 50 J/cm², so that agood blind hole wherein surface copper foil is not projected with thehole shape prevented from becoming a middle swell shape without themachining remainder of the insulation layer can be formed.Second Embodiment.

Next, a carbon dioxide laser machining method of a multilayer materialaccording to a second embodiment of the invention will be discussed withFIGS. 6A to 8. To begin with, as shown in FIG. 6B, laser light 8 withthe beam ON time being 3 μs and the energy density being 150 J/cm² isapplied to a conductor layer 4 of a wiring board as shown in FIG. 6A inarea φ50 μm smaller than area φ100 μm of the conductor layer 4 to befinally removed, and the conductor layer 4 and some of an insulationlayer 1 are removed. In the removal, the laser light with the beam ONtime ranging from 1 μs to 10 μs and the energy density being 25 J/cm² ormore is used and thus the energy of the laser light is used toefficiently remove the conductor layer 4 and extra laser light does notmachine the insulation layer 1 unnecessarily largely, so that it is madepossible to prevent the conductor layer 4 from being projected into thehole or the hole shape from becoming a middle swell shape uponapplication of one pulse.

Next, as shown in FIG. 6C, laser light 6 with the beam ON time being 3μs and the energy density being 150 J/cm² is applied to the area φ100 μmof the conductor layer 4 to be removed, overlapping the position of themachined hole in the conductor layer 4 after the laser light 8 isapplied, thereby removing the conductor layer 4 of the large area.

FIG. 7 shows on the Y axis the roundness of machined hole (=100×shortdiameter/long diameter) upon application of two pulses of laser lightdifferent in application area to copper foil 12 μm thick and shows thelaser light application area of the first pulse on the X axis. The laserlight application area of the second pulse is fixed to φ100 μm. It isseen in the figure that when the application area of the first pulse isφ60 μm or less, the roundness is improved. Generally, the roundness ofmachined hole needs to be 90% or more.

To remove the conductor layer by applying one pulse only, if theconductor surface is dirty or contains a flaw and a portion in which thelaser light absorption ratio is high exists, a temperature rise occursfrom the portion. In this case, if the portion in which the absorptionratio is high does not exist at the laser light application center, thetemperature distribution easily becomes an ellipse rather than a perfectcircle, as shown in FIG. 8B. Thus, to apply one pulse, the roundness ofthe machined hole is easily degraded.

However, to remove the conductor layer by applying two pulses differentin beam diameter, FIGS. 8A and 8B shows the temperature distribution ofa machined part upon application of laser light of two pulses or more,and a small-diameter hole is machined by applying the first pulse andthen large laser light is applied, whereby a temperature rise during thelaser light application of the second pulse occurs radially with thehole formed by applying the first pulse as the center, so that thetemperature distribution becomes a circle with the hole formed byapplying the first pulse as the center, as shown in FIG. 8A. Since theremoval phenomenon occurs in accordance with the temperaturedistribution, a machined hole having high roundness can be provided.

JP-A-9-239573 proposes a laser machining method of machining by applyinglaser light with the beam ON time ranging from 10 μs to 20 μs as thefirst pulse and machining by applying laser light of a largerapplication area than that at the first pulse with the beam ON timebeing 200 ns or less as the second pulse. However, the purpose of makingthe laser light application area of the second pulse larger than that ofthe first pulse is to remove a thermal deterioration layer and a resinremaining film in an insulation layer and the purpose of changing thebeam diameter and the beam ON time area (range shown in c in FIG. 5)also differ from those in the laser machining method of the invention.

Next, in FIG. 6C, to machine the insulation layer 1 left afterapplication of the laser light 8 and the laser light 6 (two pulses),after the laser light 6 is applied, laser light under the conditionequivalent to the laser light 7 shown in FIG. 1D is applied formachining the remaining insulation layer 1, whereby a blind hole isformed by applying a total of three pulses. In the embodiment, the laserlight of the third pulse is set to 1 μs and 50 J/cm², so that a goodblind hole having high roundness of surface wherein surface copper foilis not projected with the hole shape prevented from becoming a middleswell shape without the machining remainder of the insulation layer canbe formed.

As described above, the laser machining method according to theinvention provides the advantage that the conductor layer on the surfacecan be prevented from being projected into the hole and the hole shapecan be prevented from becoming a middle swell shape by applying laserlight to the machined part like pulses at an energy density of 25 J/cm²or more for the beam application time in the range of 1 μs to 10 μs.

INDUSTRIAL APPLICABILITY

As described above, the carbon dioxide laser machining method accordingto the invention is suited for a carbon dioxide laser machining methodof a multilayer material for forming a through hole and a blind hole toelectrically connect a plurality of conductor layers.

1. A carbon dioxide laser machining method of a multilayer material,comprising: applying carbon dioxide laser light to a machined part of amultilayer material having an insulation layer, and a first conductorlayer and a second conductor layer deposited with the insulation layerin between; and removing the first conductor layer and the insulationlayer of the machined part to form one of a blind hole and a groove to adepth of the second conductor layer, such that a portion of the secondconductor layer is not removed, wherein the laser light is applied tothe machined part in the form of pulses at an energy density of 25 J/cm²or more with a beam ON time in the range of 1 μs to 10 μs, and whereinthe first conductor layer is removed with the laser light.
 2. The methodaccording to claim 1, wherein said energy density is greater than 40J/cm².
 3. A carbon dioxide laser machining method of a multilayermaterial of applying carbon dioxide laser light to a machined part of amultilayer material having an insulation layer and a first conductorlayer and a second conductor layer deposited with the insulation layerbetween and removing the first conductor layer and the insulation layerof the machined part to form a blind hole or a groove arriving at thesecond conductor layer, characterized in that the laser light is appliedto the machined part in the form of pulses at an energy density of 25J/cm² or more with a beam ON time in the range of 1 μs to 10 μs, whereinwhen the first conductor layer of the machined part is removed byapplying a plurality of pulses, the application beam diameter of laterapplied pulses is made larger than that of earlier applied pulses.